1. Field of the Invention
The present invention concerns a radiocommunication system, typically with mobile terminals. The radiocommunication system is a satellite or terrestrial radiocommunication network, for example. To be more precise, the invention concerns a repeater adapted to provide coverage of a geographical area that is inaccessible via the radiocommunication network alone.
2. Description of the Prior Art
Global systems for mobile communications using satellites as repeaters are constantly expanding and their applications cover many fields. Initially limited to maritime applications, these systems now cover terrestrial applications. For example, tracking and paging systems are used for terrestrial transportation. New systems under development are directed to applications including the automatic tracking of cargoes or fleets (trucks, containers, rental automobiles, etc), industrial telecontrol (energy, sensors, irrigation, etc), security of goods and persons (intruder detection, rescue, tracking of materials at risk, fire alarms, floods, etc). These systems use satellites in geostationary, intermediate or low Earth orbit that relay the connections between first stations, such as connection stations, and second stations, such as mobile terminals or beacons.
In these systems, communication between a first station and a second station, for example via a satellite in a satellite network, is no longer assured if the link balance does not yield a sufficient positive margin, typically in the case of attenuation of signals due to the environment (vegetation, inclement weather, buildings, etc) or in the event of damage to the antenna (pulled off, short-circuited, etc). Loss of service occurs more or less frequently, depending on the frequencies chosen and the margins adopted in designing the system. Loss of service is typically permanent if the second station is enclosed in an underground place, such as a carpark, or a metal structure, such as the hull of a ship. It can be temporary, depending on the path followed by the second station, typically in accordance with the nature of the vegetation, the presence of obstacles such as tunnels or buildings, or in the event of non-permanent or non-total failure of the satellite repeaters.
Applications necessitating permanent tracking can be guaranteed only outside so-called "shadow areas" that are inaccessible via the radiocommunication network alone. The result is a general loss of system performance, as well as a reduced service to users. Many of these systems operate on an "interrogate and wait for response" basis. The absence of a response from a station disturbs overall operation by unnecessarily utilizing precious capacity as a result of call attempt repeats, reports, etc.
To remedy this drawback the prior art provides for the use of a repeater, which can incorporate an amplification function, to assure the coverage of a shadow area. In the GSM (Global System for Mobile communications) for example, repeaters called "boosters" are used to cover streets that are inaccessible via the base transceiver stations of the network. A repeater of this kind has a first antenna adapted to receive from and to transmit to a base transceiver station, an amplifier, and a second antenna directed towards the shadow area to be covered.
The main drawback of this prior art solution is that it does not derive any benefit from the fact that a station belongs to a shadow area, for example in terms of economizing on network capacity. The prior art solution solves the problem of the inaccessibility of a station via the network without using this inaccessibility to advantage.
The invention aims to remedy this drawback by providing a particularly advantageous repeater for radiocommunication networks.